Apparatus for measuring defects in a glass sheet

ABSTRACT

A method of measuring the topography of a large, thin, non-flat specular substrate in a production environment with minimal movement of a majority of the measurement apparatus. A gimbal-mounted reflecting element is used to steer a short coherence length probe beam such that the probe beam is substantially perpendicular to a local surface of the substrate. The probe beam and the reference beam are combined and the resulting interference pattern used to characterize defects on the local surface.

This is a divisional of U.S. patent application Ser. No. 11/708,846filed on Feb. 21, 2007, now U.S. Pat. No. 7,570,366 the content of whichis relied upon and incorporated herein by reference in its entirety, andthe benefit of priority under 35 U.S.C. §120 is hereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method of measuring defects in aglass article, and in particular, measuring the topography of thin glasssheets.

2. Technical Background

Flat panel displays, such as liquid crystal displays, are fastovertaking traditional cathode ray tube (CRT) display technology in thecommercial arena. The manufacture of LCD display devices relies on thinsheets of pristine-surfaced glass, between which a liquid crystalmaterial is sandwiched. Tolerances for surface defects for these glasssheets is extraordinarily stringent, requiring the ability to measuredefects on a nanometer scale. Exacerbating the problem is the fact thatthe glass sheets are exceptionally thin, typically less than about 0.7mm, and can be quite large—several square meters or more in someinstances. As such large, thin sheets are quite flexible, maintainingthe sheet flat, let alone stable (remaining in a given shape over aperiod of time), can be challenging.

Much effort has gone into developing appropriate fixturing, andmeasurement techniques that can measure defects quickly, thus reducingdependence on stability-related concerns (movement of the sheet overtime). The measurement devices utilized for making nanometer-scalemeasurements of substrates typically include the use of aninterferometer. Interferometers, such as the well-known Michelsoninterferometer, use interference between beams of light to create aninterference pattern indicative of the difference in optical path lengthbetween the beams. This difference in path length can be used as anindicator of the topography of a surface under measurement.

One drawback of conventional area-scan techniques for thecharacterization of nanometer-scale surface defects on a specularsurface is the need to maintain the measurement surface perpendicular tothe probe or measurement beam while keeping within the angular toleranceof the interferometer. For this reason, the sample under test may bemounted on a movable stage which may be adjusted prior to performing themeasurement. While this approach is applicable in a laboratoryenvironment, or where small sample sizes are being measured (e.g.semiconductor wafers), in a production environment for processing largesheets of very thin glass, moving the sheet becomes prohibitive:movement of the sheet can itself create distortion of the sheet surface.Moreover, alignment of the sheet may potentially require repeatedmovement of the sheet to investigate potential defects over the surface.Movement of a large sheet can require complex, bulky equipment, andincrease measurement time. Similar concerns accompany movement of theinterferometer.

What is needed is a method and/or apparatus suitable for a productionenvironment that enables nanometer-scale measurements of the surfacetopography of thin glass sheets, which may not be flat, without needlessmovement of the sheet or bulky measurement equipment.

SUMMARY

In an embodiment of the present invention a method of characterizing asubstrate is disclosed comprising providing a coherent radiation beam,providing a specular, non-flat substrate, splitting the radiation beaminto a probe beam and a reference beam, intercepting the probe beam witha first reflective element to irradiate a local surface of the substratecomprising a surface defect wherein the local surface is nominallytilted relative to a longitudinal axis of the probe beam, interceptingand reflecting the reference beam with a second reflective element,collecting the probe beam reflected from the local surface and thereference beam reflected from the second reflective element, combiningthe reference beam reflected from the second reflective element and theprobe beam reflected from the local surface to produce a first set ofinterference fringes resulting from the local surface tilt and a secondset of interference fringes resulting from the surface defect, detectingthe first and second sets of interference fringes, steering the probebeam with the first reflective element to minimize the number ofinterference fringes resulting from the local surface tilt, and usingthe detected interference fringes resulting from the surface defect tocharacterize the defect.

In another embodiment, a method of characterizing a substrate isdescribed comprising providing a radiation beam comprising a coherencelength less than about 300 μm, splitting the radiation beam into a probebeam and a reference beam, intercepting the probe beam with a firstreflective element, steering the probe beam with the reflective elementto irradiate a specular local surface of a non-flat substrate such thatthe probe beam is perpendicular to and reflected from the local surface,intercepting and reflecting with the first reflector the probe beamreflected from the local surface, intercepting and reflecting thereference beam with a second reflective element such that a nominaloptical path length of the probe beam and the reference beam aresubstantially equal, collecting the probe beam reflected from the firstreflective element and the reference beam reflected from the secondreflective element, combining the reflected reference beam and the probebeam reflected from the first reflective element to produce aninterference pattern, detecting the interference pattern across atwo-dimensional array of pixels, and using the detected interferencepattern to determine a topography of the local surface.

In still another embodiment, a method of characterizing a defect on asubstrate surface is disclosed comprising providing a coherent radiationbeam, providing a specular, non-flat substrate, splitting the radiationbeam into a probe beam and a reference beam, intercepting the probe beamwith a first reflective element to irradiate a local surface of thesubstrate comprising a surface defect wherein the local surface isnominally tilted relative to a longitudinal axis of the probe beam,intercepting and reflecting the reference beam with a second reflectiveelement, collecting the probe beam reflected from the local surface andthe reference beam reflected from the second reflective element,combining the reference beam reflected from the second reflectiveelement and the probe beam reflected from the local surface to produce afirst set of interference fringes resulting from the local surface tiltand a second set of interference fringes resulting from the surfacedefect, detecting the first and second sets of interference fringes,steering the reference beam with the second reflective element tominimize the number of interference fringes resulting from the localsurface tilt, and using the detected interference fringes resulting fromthe surface defect to characterize the defect.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate an exemplary embodiment of theinvention and, together with the description, serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of an optical system forcharacterizing a non-flat substrate.

FIG. 2 is a schematic diagram of another embodiment of an optical systemfor characterizing a non-flat substrate.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of the present invention.However, it will be apparent to one having ordinary skill in the art,having had the benefit of the present disclosure, that the presentinvention may be practiced in other embodiments that depart from thespecific details disclosed herein. Moreover, descriptions of well-knowndevices, methods and materials may be omitted so as not to obscure thedescription of the present invention. Finally, wherever applicable, likereference numerals refer to like elements.

FIG. 1 is a schematic view of a measurement apparatus according to anembodiment of the present invention based on a Michelson interferometer.Measurement apparatus 10 comprises radiation source 12, polarizing beamsplitter 14, first reflecting element 16, second reflecting element 18,collimating lens 20, aperture 22 and detection system 24.

Also shown in FIG. 1 is substrate 26. Substrate 26 is desirably flat,but due to the thinness and the large surface area of the substrate,substrate 26 tends to exhibit distortion or waviness, shown exaggeratedin FIG. 1. Substrate 26 has a first side 28 and a second side 30opposite to and substantially parallel with first side 28. Substrate 26may have less than a square meter in surface area on one of first orsecond sides 28, 30, although typically substrate 26 has a surface area(as determined on a single side surface) greater than about 1 squaremeter, and in some cases in excess of 2 square meters, and in otherinstances in excess of 10 square meters.

In accordance with the present embodiment, a short coherence length beam32 of radiation is emitted from radiation source 12 and is directedtoward polarizing beam splitter 14. Preferably, the coherence length ofthe beam is less than the thickness of the substrate being measured.Preferably, the coherence length of the radiation beam is equal to orless than about 300 μm for substrates having a thickness equal to orless than 0.7 mm. By using a radiation source which produces a radiationbeam with a very short coherence length, the apparatus can be used toeffectively image a local surface portion of a first substrate surfacewithout interference from reflections originating at the opposing secondsubstrate surface. While any radiation source capable of producing asuitable coherence length may be used, a laser has been found to bewell-suited to the present application.

Radiation beam 32 is split by polarizing beam splitter 14, with a firstportion 34 of the beam being directed toward first, steering reflector16 and a second portion 36 of the beam directed to second, referencereflector 18. The first portion 34 will hereinafter be referred to asmeasurement or probe beam 34, and the second portion will be referred toas reference beam 36. Steering reflector 16 is movable in twodimensions, being capable of both pitch and yaw movements, i.e. steeringreflector 16 is rotatable about two orthogonal axes, and movement ofsteering reflector 16 is preferably computer controlled so that theposition of steering reflector 16 may be automatically and preciselyadjusted. For example, steering reflector 16 may be mounted on a gimbal(be gimbaled). Any suitable actuator may be used for producing motion ofthe reflector. For example, reflector 16 may be actuated by servomotors, stepper motors, galvometers or other actuating methods as areknown in the art.

Alternatively, steering reflector 16 may comprise a plurality ofreflectors. For example, a first steering reflector may be rotatableabout a first axis while a second steering reflector (not shown) may bepositioned so as to intercept probe beam 34 either before or after thefirst steering reflector. The second steering reflector is rotatableabout a second axis orthogonal to the first axis. In either case, thesteering reflector or reflectors are capable of directing probe beam 34to a pre-determined location on glass sheet 26 allowable by the range ofmotion of the steering reflector(s) and within the field of view of theinterferometer. Hereinafter, the present embodiment will be described interms of a single, gimbal-mounted steering reflector, with theunderstanding that any similar reflecting device capable of redirectingprobe beam 34 to glass sheet 26 may be used. An example of a suitablegimbal-mount for the reflector is a New Focus model number 8812motorized gimbal mount. The reflecting surface itself should be capableof preserving the polarization integrity of the beams.

Following reflection from steering reflector 16, probe beam 34irradiates a predetermined location, hereinafter referred to as localsurface 38, located on first surface 28 of substrate 26, whereupon theprobe beam is reflected back to reflecting element 16 and combined withreference beam 36 in beam splitter 14. Preferably, the local surface isa specular (mirror-like) surface. The probe beam reflected from localsurface 38 is indicated by reference numeral 34 a in FIG. 1.

Reference beam 36, produced at beam splitter 14, travels to referencereflector 18 and is reflected back to beam splitter 14 where it iscollected and combined with probe beam 34. The reference beam reflectedfrom reference reflector 18 is indicated by reference numeral 36 a inFIG. 1. Combined beam 40 may then be imaged onto detection system 24comprising a suitable sensor or other detector comprising an array ofpixel elements (i.e. an m×n array of pixels). The array detector maycontain, for example, in excess of 1 million pixels in order to receive,detect and spatially resolve the fringes in combined beam 40. A CCDsensor such as those used in digital cameras is a suitable detector forinstance.

In one embodiment, detection system 24 may utilize a pixelated phaseshifting or phase mask technique, such as that disclosed in U.S. PatentPublication 2005/0046865, the content of which is incorporated herein byreference in its entirety. As described in the aforementionedpublication, a pixilated phase mask comprising sets of phase mask pixelsis used to produce a pre-determined phase shift between portions of thepolarized probe and reference beams. The resulting spatially separatedintensity patterns of each set of phase mask pixels are directed onto apixelated detector array. Advantageously, the pixilated phase masktechnique facilitates virtually instantaneous real-time characterizationof local surface 38.

In the case where local surface 38 is flat and perpendicular to theincident probe beam 34, and there are no defects on local surface 38,the combined probe and reference beams are designed to havesubstantially equal optical path lengths within the tolerance range ofthe coherence length, and therefore produce high contrast interferencefringes when combined. That is, the optical path length traversed byprobe beam 34 should be within a coherence length of the optical pathlength traversed by the reference beam 36. Consequently the combinedbeam produces broad fringes which result in a substantially constantintensity over a cross sectional area of the beam in a planeperpendicular to the longitudinal axis of the beam. That is, a singleparticular fringe may cover a substantial portion of the field of viewof the interferometer. On the other hand, if the substrate is tiltedrelative to the incident probe beam, i.e. not perpendicular to thelongitudinal axis of the incident probe beam, an interference patternwill be formed consisting of alternating light and dark fringes.

If a defect exists on local surface 38, the change in optical pathlength at the defect will incur an optical path length difference inthat portion of the beam incident on the defect. The resultant opticalpath length difference will produce a localized interference fringepattern in the combined beam and subsequent interference fringes, oralternating light and dark regions that encircle the defect in the imageof the local surface. The size (e.g. height) of the defect may bedetermined from the attributes of the interference fringes, such as bythe size and spacing of the fringes. The method of making thisdetermination is well-known in the art, and will not be describedfurther so as not to obscure the present invention.

In some cases, the local surface may not be perpendicular to thelongitudinal axis of the incident probe beam. This may occur because thesubstrate under measurement exhibits surface unevenness, or waviness,due to the large size and extreme thinness of the substrate, and animperfect mechanism for fixturing the substrate in a productionenvironment. This becomes particularly troublesome when the defects tobe measured are extraordinarily small, being on the order of nanometersin height, and the defect or defects must be measured quickly so as notto slow the production process. Waviness or other unevenness in thesheet surface may result in local surface 38 having an orientation otherthan perpendicular to the probe beam axis—the substrate may be locallytilted. Tilt of the local surface results in a differential path lengthacross the image reflected from the local surface. Tilt of the localsurface 38 is indicated by dashed line 42 in FIG. 1. When the probe beamreflected from the local surface is combined with the reference beamreflected from the reference reflector, these path length differencesresult in tilt-related interference fringes. The tilt-relatedinterference fringes may make it difficult or impossible to discern thedefect-related fringes (fringe confusion), e.g. by limiting the dynamicrange of the fringes. It is desirable therefore to have the ability tonull out, or minimize the number of tilt-related fringes in order tomeasure the defect related fringes. One method of nulling out thetilt-related fringes is by bringing the probe beam incident on thespecular local surface into alignment with the local surface. That is,direct the incident probe beam such that the longitudinal axis of theprobe beam is substantially perpendicular to the local surface. Thedegree of perpendicularity required is dependent upon the design of theinterferometer, such as, for example, the field of view of theinterferometer. For example, in some embodiments, perpendicularitywithin about 0.5 degrees of perpendicular may be applied.

Alignment of the probe beam with the local surface may be accomplished,for example, by moving measurement apparatus 10 relative to thesubstrate. However, the measurement apparatus is typically large, andheavy. It is not desirable in a production environment to be undertakingcontinuous movement of the entire measurement apparatus, as the mass ofthe instrument slows the response time for movements, and henceincreases measurement time. Moreover, the mounting apparatus necessaryto provide the requisite motion to the measurement apparatus can be morecostly than the measurement apparatus itself and require considerablephysical space to implement.

Alternatively, one may also move the substrate being measured relativeto the measurement apparatus. However, this too presents problems due tothe size and relative flexibility of the substrate: movement of thesubstrate in order to align the substrate with the probe beam may onlyfurther exacerbate the already highlighted issues, including flexing ofthe substrate which may induce greater unevenness of the substratesurface.

According to a method of the present invention, the probe beam isactively aimed or steered using reflective element 16. Reflectiveelement 16 is rotated as appropriate to ensure that probe beam 34 isincident on local surface 38 such that the tilt-related interferencefringes are minimized or eliminated. This condition is generally metwhen the longitudinal axis of the incident probe beam is substantiallyperpendicular with the local surface and the local surface is generallyflat. Although movement of reflective element 16 may result in a changein the path length of probe beam 34, as long as the movement results ina path length difference less than the coherence length of beam 32, thepath length differential is acceptable.

In another embodiment of the present invention, both the interferometerand the reflecting element are mounted on a movable stage such that thereflecting element and the interferometer are translatable such that theoptical path length of the probe beam may be varied. Thus, the secondsurface of the substrate may be imaged, with the first surface separatedfrom the substrate first surface by more than a coherence length of beam32. In a manner similar to that described supra, the topography of alocal surface on the substrate second surface may be imaged withoutinterference from the first surface, and without substantial movement ofthe substrate or measurement apparatus 10. Accordingly, by movingmeasurement apparatus 10 nominally perpendicular to first surface 28 inFIG. 1, second surface 30 may be brought into “focus”, while firstsurface 28, outside the coherence length of the beam, is not observed. Asimilar effect may be accomplished by movement of second reflector 18,without the need to move the entire apparatus. That is, second reflector18 may be translated along the longitudinal axis of reference beam 36,while maintaining the longitudinal axis of the reference beamperpendicular to the reflecting surface of reflector 18. In so doing thefield of focus of the probe beam may be varied (the field of focus inthis instance being defined as meeting the condition that the opticalpath lengths of the probe and reference beams are equal, within thecoherence length of the beams).

In still another embodiment, the probe beam may be held stationary, butwherein the reference beam is intercepted by a reflecting element whichis rotatable about at least two orthogonal axes. The basic operation ofthe reflecting element is similar to the operation of the precedingembodiment. Illustrated in FIG. 2 is an alternative embodiment ofmeasurement apparatus 10. In the apparatus of FIG. 2, first reflectingelement 16 is used to fold probe beam 34 in a predetermined directionsuch that probe beam 34 is incident on first surface 28 of substrate 26.If the measurement apparatus is oriented such that probe beam 34 isincident on substrate 26 without the need for first reflecting element16, first reflecting element 16 may be eliminated.

As shown in FIG. 2, probe beam 34 may be incident on substrate firstsurface 28 at an angle such that the longitudinal axis of probe beam 34is not substantially perpendicular to local surface 38. As a result, thetilt exhibited by the substrate imparts an angle to the reflected probebeam such that the probe beam return (reflected) path 34 a exhibits anangular deflection from the incident path 34. To compensate, secondreflector 18 is mounted on a motorized gimbal (or other suitablesteerable mount) in a manner similar to the method in which firstreflector 16 was mounted in the previous embodiment. Second gimbaledreflective element 18 is rotated about one or both of two orthogonalaxes to null the substrate tilt related interference fringes, allowingclear detection of the defect-related fringes. That is, movement ofsecond reflective element 18 causes the reference beam to be reflectedalong a path 36 a which is not coincident with the incident path 36, butwhich combines with the probe beam reflected along path 34 a to null thetilt.

It should be emphasized that the above-described embodiments of thepresent invention, particularly any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the invention. Many variations andmodifications may be made to the above-described embodiments of theinvention without departing substantially from the spirit and principlesof the invention. All such modifications and variations are intended tobe included herein within the scope of this disclosure and the presentinvention and protected by the following claims.

1. A method of characterizing a surface defect on a substratecomprising; providing a coherent radiation beam; providing a specular,non-flat substrate; splitting the radiation beam into a probe beam and areference beam; intercepting and reflecting the probe beam with a firstreflective element to perpendicularly irradiate a local surface of thesubstrate comprising a surface defect wherein the local surface istilted relative to a longitudinal axis of the probe beam; interceptingand reflecting the reference beam with a second reflective element;collecting the probe beam reflected from the local surface and thereference beam reflected from the second reflective element; combiningthe reference beam reflected from the second reflective element and theprobe beam reflected from the local surface to produce a first set ofinterference fringes resulting from the local surface tilt and a secondset of interference fringes resulting from the surface defect; detectingthe first and second sets of interference fringes; steering the probebeam with the first reflective element to minimize the number ofinterference fringes resulting from the local surface tilt; and usingthe detected interference fringes resulting from the surface defect tocharacterize the defect.
 2. The method according to claim 1 wherein athickness of the substrate is greater than a coherence length of theradiation beam.
 3. The method according to claim 2 wherein the coherencelength of the radiation beam is less than about 300 □m.
 4. The methodaccording to claim 3 wherein the thickness of the substrate is less thanabout 0.7 mm.
 5. The method according to claim 1 wherein the detectingcomprises irradiating a two dimensional array of pixels on a sensor. 6.The method according to claim 1 wherein an optical path length traversedby the probe beam is substantially equal to an optical path lengthtraversed by the reference beam.
 7. The method according to claim 1wherein the first reflective element is rotatable about two orthogonalaxes.
 8. The method according to claim 1 wherein the radiation beam is alaser beam.
 9. The method according to claim 1 wherein the substrate istransparent.
 10. The method according to claim 9 wherein the localsurface and the surface defect are on a side of the substrate oppositethe first reflective element.
 11. The method according to claim 1wherein the steering comprises aligning the probe beam such that thelongitudinal axis of the probe beam is substantially perpendicular withthe local surface.
 12. The method according to claim 1 furthercomprising translating the second reflective element parallel to alongitudinal axis of the reference beam.
 13. A method of characterizingdefects on a glass substrate comprising: providing a coherent radiationbeam; providing a specular, non-flat substrate; splitting the radiationbeam into a probe beam and a reference beam; intercepting the probe beamwith a first reflective element to irradiate a local surface of thesubstrate comprising a surface defect wherein the local surface isnominally tilted relative to a longitudinal axis of the probe beam;intercepting and reflecting the reference beam with a second reflectiveelement; collecting the probe beam reflected from the local surface andthe reference beam reflected from the second reflective element;combining the reference beam reflected from the second reflectiveelement and the probe beam reflected from the local surface to produce afirst set of interference fringes resulting from the local surface tiltand a second set of interference fringes resulting from the surfacedefect; detecting the first and second sets of interference fringes;steering the reference beam with the second reflective element tominimize the number of interference fringes resulting from the localsurface tilt; and using the detected interference fringes resulting fromthe surface defect to characterize the defect.